1. Field of the Invention
The present invention relates to an electric furnace extension method for extending an optical fiber glass body (preform) so as to obtain a predetermined outer diameter, end to an apparatus of the same.
Note that, in the present specification, an "optical fiber glass body" includes, other than the usual glass body for extension, an extension optical fiber glass body obtained by extending this glass body, i.e., a "preform".
2. Description of the Related Art
Usually, an optical fiber is produced by synthesizing an optical fiber porous glass body by a VAD process or an external CVD process, then dehydrating and sintering the optical fiber porous glass body to obtain a transparent glass body for the optical fiber, extending this to obtain an outer diameter suited for wire-drawing to form a drawn optical glass body (referred to here as a "preform"), and then wire-drawing this perform to form an optical fiber including a core and a cladding.
Heretofore, the optical fiber glass body has been drawn by heat extension of the optical fiber glass body using a burner using an oxyhydrogen flame as a heat source, that is, the burner extension method. In this burner extension method, control of the outer diameter of the extended optical fiber glass body is relatively easy, but the extension rate is slow--usually 8 to 10 mm/min.
Due to recent technical advances and along with demands for improvement of manufacturing efficiency, the outer diameter of the porous optical fiber glass body synthesized by the VAD process or external CVD process has becomes much greater than in the past. Along with this, it suffers from the disadvantage in terms of the heat in the burner extension method using a burner using an oxyhydrogen flame as the heat source in the case of an optical fiber glass body having an outer diameter before extension of more than a certain value.
For this reason, a heat extension method using a heating furnace having a larger heating capacity than a burner, more specifically an electric heater (hereinafter referred to as the "electric furnace extension method") is being adapted for optical fiber glass bodies having an outer diameter before extension of more than a certain value. Such an electric furnace extension method can increase the extension rate since the heating capacity is large. For example, in contrast to the extension rate of the burner extension method of 8 to 10 mm/min, the extension rate is 30 mm/min or more in the electric furnace extension method. For this reason, the electric furnace extension method is advantageous from the viewpoint of productivity not only for the extension of an optical fiber glass body having a large outer diameter, but also for an extension glass body having a small outer diameter.
Note that in this electric furnace extension method, since the heating range is wide, it is limited to use for vertical bodies long in the vertical direction. Accordingly, the preform is extended from top to bottom.
In this electric furnace extension method, it is necessary to connect an extension-glass-body support rod (pulling glass member) to the end portion of the optical fiber glass body or the extension glass body (preform) before extension. In the past, this connection was generally achieved in a separate step of flame extension, but it is preferable to achieve this connection in the same electric furnace extension process.
Where this connection is performed in the same electric furnace extension process, however, it suffers from the disadvantage that this connection is difficult. That is, it is necessary to align the axial center of the end portion of the glass body with the axial center of the end portion of the extension support rod (pulling use glass member), but the centers tend to be effect in connection. When the axial centers are offset in connection, the distribution of stress in the cross-section of the connection portion becomes nonuniform, so trouble such as a bending in the drawn body or breakage of the connection portion sometimes occurs.
The axial alignment at the time of extension an optical fiber glass body using an electric furnace is usually performed as follows.
The two glass members (extension use glass body and pulling use glass member), one end of each of which is held elsewhere, are inserted into the heated furnace from reverse directions to each other and made to abut against each other. The abutting ends of the two are heated and fused to each other, then the gripping member gripping the pulling use glass member is moved downward at a predetermined speed while moving the gripped portion of the extension use glass body downward at a constant speed so as to stretch the extension use glass body and extend it to the predetermined outer diameter.
FIG. 1 is a view of an example of the configuration of this type of extension apparatus for an optical fiber glass body of the related art.
In FIG. 1, 91a denotes an extension use glass body (or optical giber glass body) to be extended, and 91b denotes a dummy pulling use glass member (extension-out use glass member). Reference 93 denotes a furnace body. A furnace pipe 94 containing an electric heater is disposed in the furnace body 93.
One end portion of each of the extension use glass body 91a and the pulling use glass member 91b is gripped and affixed by gripping members 95 and 96 comprising for example three-class chucks. The gripping members 95 and 96 can be moved by a moving stand 97 to which the gripping members 95 and 96 are affixed in the vertical direction in a state illustrated in FIG. 1. Reference numeral 98 denotes a guide rail for guiding the moving stand 97.
The optical fiber glass body is extended (stretched) by the above optical fiber body extension apparatus as follows:
(1) The fixed ends of the extension use glass body 91a and the pulling use glass member 91b are gripped and affixed by the gripping members 95 and 96, respectively. The front ends of the free end portions of the affixed extension use glass body 91a and pulling use glass member 91b are axially aligned, then the two are moved into the furnace pipe 94 and made to abut against each other. PA1 holding and fixing said fix end of the lower portion of said extension optical fiber glass body by said first holding member; PA1 releasing said pair of axial center holding members of said second axial center alignment mechanism and positioning the tip or the free end of said pulling glass member, to thereby coincide the tip of the free end of said extension optical fiber glass body and the tip of the free end of said pulling glass member to meet each axial center. PA1 holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member; PA1 holding a tip of a free end of a lower portion of said extension optical fiber glass body by said first axial center alignment mechanism and aligning an axial center of said tip of said free end; PA1 holding and fixing said fix end of the lower portion of said pulling glass member body by said first holding member; PA1 releasing said pair of axial center holding members of said first axial center alignment mechanism and positioning the tip of the free end of said extension optical fiber glass body at a predetermined position is said electric furnace; to thereby coincide the tip of the from end of said extension optical fiber glass body and the tip of the free end of said pulling glass member to meet each axial center.
(2) The furnace pipe 94 is heated to the predetermined temperature to heat and fuse the abutted end surfaces of the two to join them, then the gripping member 95 gripping the pulling use glass member 91b is moved downward to extend the extension use glass body 91a to the predetermined outer diameter.
(3) The axial alignment of the extension use glass body 91a and the pulling use glass member 91b is carried out when heating and fusing the abutted end surfaces of the two in the furnace pipe 94 by the extension apparatus. For this axial alignment (centering), the fixed end portions of the extension use glass body 91a and the pulling use glass member 91b are for example moved delicately by hand within the range of free play of the gripping members 95 and 96, for example, the three-claw chucks, to center the axial centers of the front ends of the free end portions, then are fixed in place. The fixed front ends of the free end portions are then made to abut in the furnace pipe 94. The state of axial alignment is confirmed visually from an observation window 94A provided in the furnace pipe 94.
This centering method of the related art takes a long time. Also, also the precision of alignment is not very good. For example, an axial deviation of .+-.1 to .+-.2 mm (5 to 10% of the diameter of the glass member) occurs in the horizontal plane. If the precision of the axial alignment is poor, the distribution of stress at the cross-section of the connection portion of the extension use glass body 91a and the pulling use glass member 91b with become nonuniform at the time of extending, and trouble such as occurrence of bending in the extended fiber glass body or breakage of the connection portion may occur.
Further, where the optical fiber glass body is extended by the electric furnace extension method explained above, bending or waviness sometimes occur in the extended fiber glass body (preform) or variation sometimes occurs in the outer diameter, so the outer diameter sometimes deviates from the prescribed value.
Further, improvement of the extension rate of preforms has also been demanded from the veiwpoint of productivity.